1. Field of the Invention
Embodiments of the invention relate to a temperature sensor. More particularly, embodiments of the invention relate to a temperature sensor capable of linearly changing a sensing temperature.
2. Description of the Related Art
A temperature sensor is commonly applied to applications and systems incorporating one or more semiconductor devices in order to sense a peripheral temperature. As the peripheral temperature varies, the operating conditions of the circuit blocks included within the semiconductor integrated circuit are often modified in some controlled manner. For example, the dynamic random access memory (DRAM) used in mobile products may modify its refresh period in accordance with variation in peripheral temperature.
Figure (FIG.) 1 is a circuit diagram illustrating a conventional temperature sensor.
Referring to FIG. 1, the temperature sensor includes a reference voltage generator 10 and differential amplifiers 20, 30 and 40. The reference voltage generator 10 is a band-gap reference voltage generator well known to one of ordinary skill in the art.
A reference current IR is determined by a loop composed of NMOS transistors MN1 and MN2, a resistor RR and diodes D1 and D2. When the size ratio of diode D1 to diode D2 is M:1, a current flowing through the resistor RR, i.e., the reference current IR, may be represented by the following expression:IR=k·T/q·ln(M)/RR, where ‘k’ denotes Boltzmann's Constant, ‘T’ denotes absolute temperature and ‘q’ denotes quantity of electric charge of an electron.
That is, the reference current IR increases proportionally to the absolute temperature T. The gate of the NMOS transistor MN2 and the gate of an NMOS transistor MN3 are coupled to an identical bias voltage, and the gate bias voltage decreases as the peripheral temperature increases. As a result, a current IA that flows through resistors R1 and R2 decreases proportionally with the absolute temperature.
FIG. 2A shows relationships between the currents IR and IA, and the peripheral temperature. As may be seen from these graphed relationships, the reference current IR increases as the peripheral temperature increases, and the current IA decreases as the peripheral temperature increases.
FIG. 2B shows relationships between a reference voltage OREF and a temperature sensing voltage OTA, and the peripheral temperature. Referring to FIG. 2B, the reference voltage OREF and the temperature sensing voltage OTA both increase as the peripheral temperature increases, but have different slopes. As may be seen in FIG. 2B, a graphical plot of the reference voltage OREF and temperature sensing voltage OTA as a function of temperature intersect at a single point. This point indicates a sensing temperature TS.
Referring back to FIG. 1, the reference voltage generator 10 outputs the temperature sensing voltage OTA and the reference voltage OREF to the differential amplifiers 20 and 30. The differential amplifier 20 amplifies a voltage difference between the temperature sensing voltage OTA and the reference voltage OREF to output a first differential output signal DIF01.
When the level of the temperature sensing voltage OTA becomes higher than that of reference voltage OREF, the output voltage TOUT increases, and when the level of the temperature sensing voltage OTA becomes lower than that of reference voltage OREF, the output voltage TOUT decreases. Thus, an increase in the output voltage TOUT indicates that the peripheral temperature is higher than a currently set sensing temperature, and a decrease in the output voltage TOUT indicates that the peripheral temperature is lower than the currently set sensing temperature.
As shown in FIG. 1, the conventional temperature sensor controls the sensing temperature by controlling each resistance value of resistor strings R1 through R6. Each of the resistance values of the resistor strings R1 through R6 is controlled by switching NMOS transistors MN4, MN5, MN6 and MN7 that are coupled in parallel with the resistors R3, R4, R5 and R6.
However, when the sensing temperature is adjusted by controlling the resistance value, the sensing temperature is nonlinearly varied in accordance with variations of the resistance value. Accordingly, the conventional temperature sensor shown in FIG. 1 is mainly used to test the temperature sensor in two, selected temperature conditions.
This limited ability is inadequate for emerging applications that require more precise sensing and corresponding control over the operation of semiconductor devices in response to variations in peripheral temperature. In sum, a temperature sensor capable of variously (including linearly) changing a sensing temperature is required.